Poly-crystalline, as well as mono-crystalline, diamond has been grown using variety of CVD techniques. Poly-crystalline diamond, in spite of having similar properties as mono-crystalline diamonds, is not a potential material for new applications.
For example, thermal conductivity of the poly-crystalline diamond still does not surpass thermal conductivity of natural diamond. Indeed, in poly-crystalline diamond, the grain boundaries inhibit exhibition of superior properties unique to diamond because the grain boundaries act as scattering centres for phonons thereby deteriorating thermal and other properties. The presence of large angle as well as small angle grain boundaries are a major drawback in applications of poly-crystalline diamond.
While there is a clear preference for using mono-crystalline diamonds in applications, mono-crystalline diamonds are difficult to grow with the same texture, clarity, purity and finish as natural diamond. Although, mono-crystalline diamond has superior properties compared to poly-crystalline diamond, microscopic and macroscopic graphitic and non-graphitic inclusions, feathers (long line defects) are very common in CVD grown mono-crystalline diamond. As a result, the potential of CVD grown mono-crystals of diamond to be used as a gem quality product is diminished.
Detailed characterization of defects in mono-crystalline CVD grown diamond by Raman spectroscopy and X-ray diffraction (XRD) reveals that the defects comprise graphitic regions having a size in the range of submicrons and several microns in otherwise mono-crystalline diamond.
Another difficulty in growing mono crystalline CVD diamond is the growth rates. Although the growth rates of 70-100 microns per hour are possible with addition of nitrogen to CVD gases, but defects are prevalent and generally defect density increases with the growth rate.
For example, Derwent abstract of Japanese publication number JP 07277890 discloses a method for synthesizing diamond for use as semi-conductor, electronic or optical components or use in cutting tools. Specifically, the method disclosed in JP 07277890 involves growing diamond in the presence of gas containing nitrogen in a ratio of nitrogen to hydrogen of 3 to 1000 ppm or containing oxygen in a ratio of oxygen to carbon of 3 to 100% to increase growth rates.
A technical paper by Yan et. al. (PNAS, 1 Oct. 2002, Vol. 99, no. 20, 12523-12525) discloses a method or producing mono-crystalline diamond by microwave plasma chemical vapour deposition (MPCVD) at growth rates in the range of 50 to 150 microns per hour.
The method involves a CVD process carried out at 150 torr and involves adding nitrogen to CVD gases to provide a ratio of nitrogen to methane of 1 to 5% N2/CH4. Yan et. al. believe that nitrogen in the stated ratio enhances growth rates because more available growth sites are created. This is believed to be a result of causing growth to change from <111> crystal planes to <100> crystal planes.
The importance of nitrogen content in CVD gases is recognised in U.S. Pat. No. 5,015,494 (Yamazaki) which teaches a method of growing diamond with customized properties for dedicated applications.
Yamazaki discloses forming diamond by electron cyclotron resonance CVD and discloses adding nitrogen to “prevent lattice defects from growing by virtue of external or internal stress”. Nitrogen is added in a ratio of nitrogen-compound gas to carbon-compound gas of 0.1 to 5%. The resultant diamond has a nitrogen concentration of 0.01 to 1 wt %.
Additionally, Yamazaki discloses a requirement to add boron gas to the CVD gases to form boron nitride which deposits on a substrate to improve adhesion to the substrate of formed diamond.
Nitrogen, according to Yan et. al. and Yamazaki, is required for two purposes. Specifically, nitrogen is used to enhance growth rates of CVD grown mono-crystalline diamond and to prevent lattice defects in electron cyclotron resonance CVD grown mono-crystalline diamond.